Questions and Answers
What are alpha rays? How are they produced?
Alpha "rays" are actually high speed particles. Early researchers tended to refer to any form of energetic radiation as rays, and the term is still used. An alpha particle is made up of two protons and two neutrons, all held together by the same strong nuclear force that binds the nucleus of any atom. In fact, an alpha particle really is a nucleus - it's the same as the nucleus of a common atom of helium - but it doesn't have any electrons around it, and it's traveling very fast. Alpha particles are a type of ionizing radiation.
To describe the production of alpha particles, we have to define radioactive decay. This process can be thought of as follows. Certain combinations of neutrons and protons in a nucleus are stable. For example, in a stable bismuth atom there are 83 protons and 126 neutrons. This is called bismuth-209 (126 + 83 = 209). It will always be bismuth-209*. But if we were to add one more neutron to this atom, and make it bismuth-210, it would now be unstable, or radioactive. The atom will eventually spontaneously change or "decay", to become more stable. There are only certain ways it can do this. One way is to emit an alpha particle. In this transition, it spits out a piece of itself (the alpha particle), and becomes more stable. The alpha particle is the radiation given off during the process of "alpha decay". Since it lost two protons and two neutrons, the old bismuth atom is now an atom of thallium-206. Now, this thallium is more stable, but is also radioactive. It will decay again (but not by alpha decay), this time becoming a completely stable atom of lead. Only relatively "heavy" atoms - like bismuth - can go through alpha decay. Lighter radioactive elements go through other types of transitions to become stable. There are plenty of these radioactive materials naturally present on the Earth, which is how these radiations were discovered.
Another way to produce alpha particles is to "force" an atom to emit one. This is done by taking advantage of certain properties of various atoms. Here's an example. If we take some regular atoms of boron-10 (five protons, five neutrons), and expose this boron to a field of slow-moving neutrons, some of the boron atoms will absorb a neutron. When this happens, the outcome is not what you'd expect. The boron-10 does not just become stable boron-11. A likely possibility is that the "excited" boron atom will emit an alpha particle, becoming stable lithium in the process. There are other atoms that behave in this fashion.
Although alpha radiation travels very fast, it can easily be blocked or shielded. Alpha particles have an electric charge because of the protons. As they move through matter, they are constantly interacting with other charged particles, such as electrons. This process transfers the motion (energy) of the alpha particle to the electrons, actually knocking the electrons free in the process. This is known as ionization. These interactions cause the alpha particle to lose its energy and come to rest. Imagine a cue ball as it is traveling along on a pool table, running into other billiard balls and eventually stopping. With alpha particles, this happens in a very short distance, even in air. Alpha particles will loose all their energy in just a couple inches of travel in air. Once an alpha particle is stopped, it grabs the first two free electrons it can find, and becomes a plain old atom of helium.
Alpha radiation is not hazardous if the source is external to the body. Alpha particles don't penetrate deeply enough into the body to reach living tissue. If the source of the alpha radiation is internal to the body, then the ionization we mentioned earlier can damage living tissue. So, safety practices for handling alpha-emitting materials are centered on preventing inhalation or ingestion of the material.
For a huge listing of information about radiation, see The Radiation Information Network.
*[Editor's Note - On April 23, 2003, it was reported that bismuth-209 is not a stable isotope, but decays into thallium-205 through alpha decay. Bismuth-209 has an extremely long half-life, roughly 1.9×1019 years. More information can be found here.]